Flexographic inking systems are used for printing a wide variety of substrates both porous such as paper and paperboard and non-porous such as polymer films and foils and are based on a two or three-roll system or a system that uses enclosed doctor blades. The three-roll system uses a rubber pickup roll that picks up the ink and transfers it to an anilox roll (engraved metal or ceramic roll). The ink fills the engraved cells and is then transferred to plates (either rubber or plastic), where the image is raised in relief from the background, non-image area. A final transfer then occurs from the plate to the substrate. Flexographic presses run at speeds in the range of 0.25 to 10.2 m/s.
Flexographic inks may be water- or solvent-based and are suitable for overprinting colour on colour, water-based inks typically having a pH of at least 8.5. Flexographic inks are known as liquid inks because of their low viscosity: 10 to 100 mPa.s under printing conditions. Prints produced by flexography can fulfil a wide range of functional effects other than the provision of information such as antistatic properties, e.g. for packaging for electronic components sensitive to electrical discharges, and electroconductive properties.
Flexographic inks containing electroconductive agents are reported in the patent literature. These are based upon metal particles, see for example WO 00/29208, U.S. Pat. No. 5,189,952, U.S. Pat. No. 4,221,830, U.S. Pat. No. 4,282,692, U.S. Pat. No. 5,286,415, U.S. Pat. No. 5,389,403 and U.S. Pat. No. 5,656,081, or carbon blacks particles, see for example JP 49-068807A, WO00/77408, U.S. Pat. No. 5,286,415 and U.S. Pat. No. 5,389,403. Metallic inks are made from aluminum or bronze powders mixed with a suitable resin soluble. Such inks cannot be integrated into colour work due to the non-transparency of the prints made therewith.
Flexographic inks containing electroconductive agents capable of producing prints which can be incorporated into the informational attributes of flexible packaging, but also fulfilling an additional functional role are therefore required.
WO 99/34371 discloses a silk screen paste with a viscosity of 1 to 200 dpa.s (102 to 2×104 mpa.s) containing a solution or dispersion of a conductive polymer paste and optionally binders, thickeners and fillers. WO 99/34371 further discloses a process for preparing silk screen pastes in which a solution or dispersion with a content of <2% by weight of poly(3,4-ethylenedioxythiophene) [PEDOT]/poly(styrenesulfonate) [PSS] is concentrated to a solids content of >2% by removing the solvent and in which subsequently binder and/or filler are optionally added. Example 1 discloses evaporation of water from a 1.3% by weight solids dispersion of PEDOT/PSS in water to a 3% by weight solids dispersion in a rotary evaporator at 45° C. and 20 mbar.
EP-A 1 081 549 discloses a coating composition comprising a solution of a substituted or unsubstituted thiophene-containing electrically-conductive polymer, a film-forming binder, and an organic solvent media; the media having a water content of less than 37 weight percent. Coating dispersions with 0.1% by weight of PEDOT/PSS, i.e. 0.0294% by weight of PEDOT since BAYTRON™ P contains a weight ratio PEDOT to PSS of 1:2.4, and with between 8 and 25% by weight of water were disclosed in the invention examples using BAYTRON™ P, a 1.22% by weight dispersion of PEDOT/PSS in water, as the starting material.
EP-A 1 081 546 discloses a coating composition comprising a solution of an electrically-conductive polymer and an organic solvent media wherein the solvents are selected from the group consisting of alcohols, ketones, cycloalkanes, arenes, esters, glycol ethers and their mixtures; the media having a water content of less than 12 weight percent. Coating dispersions with PEDOT/PSS concentrations between 0.02 and 0.1% by weight, i.e. between 0.00588 and 0.0294% by weight of PEDOT since BAYTRON™ P contains a weight ratio PEDOT to PSS of 1:2.4, and with between 2 and 8% by weight of water were disclosed in the invention examples using BAYTRON™ P, a 1.22% by weight dispersion of PEDOT/PSS in water, as the starting material.
EP-A 1 081 548 discloses a coating composition comprising a substituted or unsubstituted thiophene-containing electrically-conductive polymer and an organic solvent media; the media having a water content of less than 12 weight percent. Coating dispersions with PEDOT/PSS concentrations between 0.02 and 0.1% by weight, i.e. between 0.00588 and 0.0294% by weight of PEDOT since BAYTRON™ P contains a weight ratio PEDOT to PSS of 1:2.4, and with between 2 and 8% by weight of water were disclosed in the invention examples using BAYTRON™ P, a 1.22% by weight dispersion of PEDOT/PSS in water, as the starting material.
EP-A 593 111 discloses a sheet or web material comprising a water resistant support or paper support coated with at least one water resistant resin layer wherein said resin support or resin layer is coated with an outermost antistatic layer containing as an essential component a polythiophene with conjugated polymer backbone in the presence of a polymeric polyanion compound, characterized in that said antistatic layer contains also a hydrophobic organic polymer having a glass transition value (Tg) of at least 40° C., said polythiophene being present at a coverage of at least 0.001 g/m2 and the weight ratio of said polythiophene organic polymer being in the range if 1/10 to 1/1000.
EP-A 821 025 discloses a thermal and mechanical formable polymer foil characterized by a transparent antistatic layer of a mixture of 3,4-polyethylenedioxythiophene with a thermally or mechanically formable binder.
WO 01/65903 discloses a method of forming an EMI shield comprising the steps of: (a) providing a thermoformable film comprising a first side and a second side; (b) applying an extensible conductive coating to the thermoformable film by printing or coating processes comprising flexographic printing, screen printing, gravure printing, offset printing, letter press printing, pad printing, slot coating, flood coating, spray coating and jet printing; (c) cutting the thermoformable film into a three-dimensional shape; and (e) applying a compressible EMI gasket to the thermoformable film, wherein steps (b) through (e) may be performed in any order.
Commercially non-transparent conductive flexographic inks on the basis of carbon black, e.g. THERMOKETT™ Conductive Ink and Black VX7011 from Akzo Nobel Inks, and metallic pigments are available. There is a need for flexographic inks capable of producing transparent conductive prints.